The present invention relates generally to additives for acid pickling baths, and more particularly to an improved additive for hydrochloric acid pickling baths utilized for batch and continuous pickling of all types of steel that accelerates the pickling rate, which reduces raw acid consumption, and that increases the brightness of the pickled steel.
Hot-mill surface scale on steel generally consists of outer layers of iron oxides having high oxygen contents, such as Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, an inner layer of iron oxides having low oxygen content, such as FeO, and possibly intermediate layers of mixed iron oxides. In addition to iron oxide scale, the steel may also have surface dirt. Pickling is generally understood to be the chemical removal of the surface scale and dirt with mineral acids, such as sulfuric, hydrochloric, and nitric-hydrofluoric acids.
When iron oxides dissolve in mineral acid pickling baths, the ferrous salt and water are formed. In hydrochloric acid pickling baths, the reactions of the acid with the iron oxides are: ##STR1## When metallic iron reacts with mineral acids, the ferrous salt and hydrogen are formed. In hydrochloric acid pickling baths, the reaction of the acid with the metallic iron is: EQU 2HCl+Fe.fwdarw.FeCl.sub.2 +H.sub.2.
Inhibitors are agents that are added to mineral acid pickling baths to protect the exposed surface of the metallic iron. They do so by inhibiting or retarding the reaction of the acid with the metallic iron without affecting, to an appreciable degree, the pickling action that is removing the iron oxide scale and dirt from the surface. If mineral acid baths were used for pickling without inhibitors, iron oxide scale would be removed, but the acid attack upon the metallic iron would be rapid and severe, depending upon the acid concentration and the pickling bath temperature. Pitting of the metal surface would occur, undesirable changes in the physical properties of the metal could take place, and carbon "smut" could be deposited on the surface. Undesirably large amounts of hydrogen gas would also be generated and released into the surrounding atmosphere, resulting in increased corrosion of all metal in the area of the pickling line. The amount of raw acid consumed would be large, and the volume of waste acid generated would increase. For these reasons, inhibitors are typically added to mineral acid pickling baths.
An effective inhibitor must disperse throughout the pickling bath in low concentrations, it must suppress hydrogen evolution, and it must not leave a smudge or film on time surface of the metal. It must also maintain constant effectiveness over a range of acid and iron concentrations and temperatures.
Accelerators are agents that are added to mineral acid pickling baths for the purpose of lowering the interfacial tension between the surface of the metallic iron and the mineral acid pickling baths to enable the pickling baths to remove iron oxide scale and surface dirt more effectively and for longer periods of time. The mineral acid pickling accelerator additives of the prior art have lengthened the working lives of mineral acid pickling baths, acid consumption has consequently decreased, and the costs of waste pickling acid neutralization and disposal have correspondingly decreased.
State of the prior art mineral acid pickling inhibitors and accelerators are commerically available from Crown Technology, Inc., 7513 E. 96th Street, P.O. Box 50426, Indianapolis, Ind. 46250-0426. The Crown L-60BNF brand non foaming hydrochloric acid inhibitors; the Crown ACID AID 5LXS-IHNF brand non foaming, hydrochloric acid pickling accelerator with a Crown L-60BNF inhibitor in the formulation; and the Activol 1803 brand inhibitor utilized in the comparative tests with the novel hydrochloric acid pickling accelerator of the present invention that follow, are proprietary mixtures of polyethylenepolyamine, coco amine, methenamine, and a proprietary fluorinated surfactant. ACID AID is a Trademark of Crown Technology, Inc., and is Registered upon the Principal Register of the United States Patent and Trademark Office.